1. Field of the Invention
The present invention relates to an OFDM receiver apparatus that receives an OFDM (Orthogonal Frequency Division Multiplexing) signal and demodulates it. An OFDM system is one of multi-carrier modulation techniques for modulating data to be transmitted by dividing it into a plurality of sub-carriers. The OFDM system improves a multipass fading resistance property because each sub-carrier becomes a narrow-band. On the other hand, in the OFDM system, property degradation due to a phase rotation caused by a carrier frequency error, a clock displacement and a phase noise increases. For this reason, an estimation and compensation of phase distortion are necessary.
2. Description of the Related Art
According to the format of the OFDM signal, a synchronizing preamble to use in a synchronous process and a preamble to use for a channel estimation are arranged sequentially in beginning of a burst-shaped OFDM signal. The preambles are followed by data. The data is formed of one or more symbols. Each symbol comprises a plurality of sub-carriers. Several of the sub-carriers is known signal pilot sub-carriers, and the remaining ones thereof are data sub-carriers including data.
Japanese Patent Laid-Open No. 2000-286819 (patent document 1) discloses an example of an OFDM receiver which receives and demodulates such an OFDM signal. In this conventional OFDM receiver, the received burst OFDM signal is converted into a digital baseband signal, and subjected to a time synchronization process and a frequency synchronization process. Then, the signal is divided into a plurality of sub-carrier signals by a Fourier transformation. The sub-carrier signals are subjected to distortion compensation with an equalizer, and then sent to a demodulator to be demodulated. Phase error information common to each sub-carrier is generated using channel distortion information generated by the signal subjected to a Fourier transform and phase distortion information generated by the channel distortion information and demodulated data. Further, the phase error information is subjected to a moving average. An equalization control signal used for distortion compensation is generated with an equalizer, using the moving averaged phase error information and channel distortion information.
The equalizer carries out an equalization process including phase distortion compensation and channel distortion compensation. An estimation of the phase distortion that could not be compensated by the equalization process is done by means of a signal generated by the equalization process, to generate the above-mentioned phase distortion information. According to the patent document 1, in an estimation process of phase distortion, the phase distortion information is weighted every sub-carrier according to amplitude level information of each sub-carrier that is calculated based on the channel distortion information. The weighted phase distortion information items are synthesized. This weighted and synthesized phase distortion information is subjected to a moving average over a plurality of symbols.
As described above, in the conventional OFDM receiver, the phase distortion that could not be compensated with an equalizer was estimated using the phase-distortion compensated signal. Then, the phase distortion information is subjected to moving average over a plurality of symbols. As a result, estimation precision of phase distortion is improved. In such an OFDM receiver, when the phase distortion estimation precision deteriorates temporarily, the estimation precision degradation may occur for a long term after the phase distortion estimation by the moving average.
It is an object of the present invention to provide an OFDM receiver apparatus realizing highly precise phase distortion compensation.